1. Field of the Invention
This invention relates to an electronic control device for internal-combustion engines and more particularly to an electronic control circuit adapted to realize driving stability without slow-speed hunting during travel at a low vehicle speed wth a throttle valve in a fully closed operating point.
2. Description of the Prior Art
Generally, in a motor vehicle travelling at a low speed with a throttle valve at a fully closed operating point, there takes place unpleasant low-frequency vibration caused by the rotation of an engine and accompanying longitudinal vibration of the motor vehicle. This is termed slow-speed hunting.
FIG. 1 shows an electronic control device for internal-combustion engines. Referring to FIG. 1, prior-art devices (for example Laid-Open Japanese Pat. No. Sho 61-145332) will be described. In this drawing, 1 is an engine; 2 is a piston; 3 is a cylinder; and 4 is a cylinder head. To an exhaust port 5 of each cylinder of the cylinder head, an exhaust manifold 6 is connected; and to an intake port 7, an intake manifold 8 is connected. In the intake manifold 8 a surge tank 9 is provided to prevent intake air pulsation; and in the surge tank 9 an intake pressure sensor 10 is provided to sense a pressure, or an intake pipe pressure Pm, in the intake manifold 8. A numeral 11 is a throttle valve which controls the quantity of intake air to be fed into each cylinder through the surge tank 9; 12 is an idle speed control valve (ISCV) which controls the quantity of intake air flowing through a bypass passage 12A bypassing the throttle valve 11; and 13 is an intake temperature sensor which senses intake air temperature. To the throttle valve 11 is directly connected a throttle position sensor 14 having a throttle valve opening sensor which outputs a signal in accordance with the amount of its opening and an idle switch which is on when the engine 1 is idling. A numeral 15 is an oxygen concentration sensor which is mounted in the exhaust manifold 6 to sense oxygen concentration in exhaust gases; 16 is a water temperature sensor which senses cooling water temperatures of the engine 1; 17 is a distributor which applies a high voltage output from an igniter 19 at a specific timing to an ignition plug 18 of the engine 1; 20 is a speed sensor which is mounted in the distributor 17 and gives off a pulse signal correspondingly to the number of revolutions Ne of the engine 1; 21 is a starter sensor which senses the operating condition of a starting motor not illustrated which starts the engine 1; 22 is an air conditioner switch which senses the operating condition of a compressor for an air conditioner and 23 is a vehicle speed sensor which is mounted on a driven wheel for sensing the running condition of a motor vehicle and senses its number of revolutions.
Various sensing signals from the aforementioned intake air pressure sensor 10, intake air temperature sensor 13, throttle position sensor 14, oxygen concentration sensor 15, water temperature sensor 16 and speed sensor 20 are output to a control circuit 25, by which various controls such as the control of the quantity of fuel injected from the fuel injection valve 26 and the control of the injection timing of the spark plug 18 are effected according to the aforementioned sensing signals.
Next, the operation of the prior-art device described above will be explained by referring to the flowcharts of FIGS. 2 and 3. FIG. 2 shows a program for sensing a slow-speed hunting state of a motor vehicle, that is, unpleasant low-frequency vibration caused by the rotation of the engine 1 and longitudinal vibration of the motor vehicle taking place along with it. This routine is executed by a CPU in the control circuit 25 by each specific number of revolutions of the engine 1 or by each specific time. First, at Step 301, a decision is made on whether or not the fuel is cut off; at Step 302, a decision is made on whether or not the throttle valve 11 is fully closed; at Step 303, a decision is made on whether or not the number of revolutions of engine Ne is below the specific value (1000 rpm); and at Step 304, a decision is made on whether or not the vehicle is travelling at a slow speed; each by using the output of the throttle position sensor 14, the speed sensor 20, and the vehicle speed sensor 23. The above-mentioned conditions decided will be established and the condition decision routine will proceed to Step 305 when fuel injection is being effected, the throttle valve 11 is fully closed, the number of revolutions Ne is Ne&lt;1000 (rpm), and when the vehicle is travelling at a slow speed of over 2.5 km/h and under 8 km/h. At Step 305, the flag X is set at "1" to indicate the decision of the conditions. On the contrary, if any one of the aforementioned conditions is not decided, the routine proceeds to Step 306, and the flag X will be reset to "0".
After the sensing of a slow-speed hunting state by the decision routine in FIG. 2, the following processing is carried out on the basis of a result of flag X setting.
The routine shown in FIG. 3 is performed by the control circuit 25 by producing a control power to the igniter 19 prior to causing the spark plug 18 to produce a spark. First, Step 401 is executed, operating and computing the ignition timing .theta. presumed to be optimum for the engine 1 through ordinary ignition timing control on the basis of the output of various sensors shown in FIG. 1. Next, at Step 402, the decision of the flag X is accomplished; when the flag X is "1", proceed to Step 403, where a specific ignition timing (in this case, 10.degree. BTDC) is stored to the value .theta.R of the ignition timing to be actually executed. Thus, a signal to be output to the igniter 19 is as to be equal to the value .theta.R, is controlled by the ignition execution routine not illustrated which the control circuit 25 executes at a specific crank angle. On the other hand, when the flag X is "0", proceed to Step 404, where the ignition timing .theta. determined by such parameters as the intake air pressure Pm computed at Step 401 and the number of revolutions of engine Ne is stored, as it is, to .theta.R.
As described above, when the flag X is "1", the ignition timing is fixed at 10.degree. BDTC without regard to the running state of the engine 1. Repeating the above-mentioned operation restrains rotational variation to prevent slow-speed hunting.
Conventional electronic control devices for internal-combustion engines are of the constitution described above, and have such a problem that when the engines have been judged to be in a slow-speed hunting state, the ignition timing is fixed at 10.degree., and if the slow-speed hunting condition is not satisfied, the ignition timing .theta. is adopted, and therefore the ignition timing will suddenly vary, giving a shock to the motor vehicle.